1. Field of the Invention:
This invention relates to an improvement in the brightness and contrast of a projection display apparatus for projecting images, which are produced on a light valve, enlarged onto a screen.
2. Description of the Related Arts:
Referring to FIG. 15 of accompanying drawings, a description will be given of a conventional projection display apparatus. In FIG. 15, reference numeral 1 designates a light source; while 9 and 10 designate a reflecting mirror and a lamp, respectively both of which constitute the light source 1; 2, represents a luminous flux emanated from the light source 1; 3, represents a polymer dispersed liquid crystal light valve; 5, represents a lens; 6, a diaphragm; 4, a projection lens; and 7, a screen.
The operation of the projection television apparatus in such a conventional device will now be described. The light source 1 irradiates a luminous flux 2 consisting of parallel light beams towards the light valve 3. A discharge bulb such as a metal halide lamp, a xenon lamp, or a halogen lamp, for instance, is generally used as the lamp 10 of the light source. Upon the surface of the light valve 3 is displayed an image, as will be described later, and the luminous flux incident on the inside of the light valve 3 is transmitted, or is dispersed dependent on the density and color of the image. The luminous flux depicted in solid lines emitted in parallel from the light valve 3 is converged at the diaphragm 6 by means of the lens 5, and then thus converged luminous flux is incident on the projection lens 4 after having passed through the diaphragm. Meanwhile, luminous flux 8, having been dispersed at the light valve 3 and passed through the lens 5, is cut off by the diaphragm 6, and hence the dispersed light will not enter the projection lens 4. Practically, the diaphragm 6 cuts off unwanted light, i.e., scattered light, and allows only the luminous flux emitted at right angles from the light valve 3 to travel to the projection lens 4, thereby improving the contrast of the image. The luminous flux having passed through the projection lens 4 is projected onto the screen 7 in the form of an enlarged image for entertainment purposes.
Next, the structure and operation of the conventional polymer dispersed liquid crystal light valve 3 will be described with reference to FIGS. 16 and 17. Liquid crystals 32 are dispersed in the form of droplets in the interior of a matrix composed of polymer 33, and such liquid crystals are sandwiched between two glass substrates 31a and 31b. The liquid crystal 32 together with the polymer 33 is hereinafter referred to as a polymer dispersed liquid crystal. Without the application of voltage (V=0) as shown in FIG. 16, the liquid crystals 32 in the droplets are arranged disorderly. In such a disordered state, there occurs a difference in refractive index, and incoming light turns into scattered light 2b. However, when a voltage V greater than a threshold voltage Vth is applied as shown in FIG. 17, the liquid crystals 32 become aligned in one direction. In the case where the liquid crystals 32 are arranged in one direction, if the refractive index is adjusted to the same refractive index as the polymer 33 in advance, the incoming light 2 will not undergo diffraction and becomes transmitted light 2a. With the increase of the voltage, the accuracy of the alignment of the liquid crystals 32 is improved, whereby the quantity of light transmitted is increased.
Referring to FIG. 18, the structure of the conventional light valve 3 is now explained. In FIG. 18, reference numeral 34 is a pixel; 35, a switching element; 36, a source electrode; and 37, a gate electrode. With such a structure, as is publicly known, the selection of the source electrode 36 and gate electrode 37 enables any desired pixel 34 to be turned on and off with the use of the switching element 35. As can be seen from the drawing, the array of electrodes addressed two-dimensionally constitute an image display element in two dimensions. Red (R), green (G), and blue (B) pixels are arranged in a triangular arrangement shown in FIG. 18. In order to make the pixels transmissive for three primary colors, a color filter for transmitting light is disposed for each pixel as conventionally practiced. A drawing of the color filter is omitted here for clarity.
In the conventional projection display apparatus having the structure as mentioned above, it was necessary to reduce the aperture diameter of the diaphragm 6 disposed right in front of the projection lens 4 to increase the contrast ratio. This, however, resulted in partial elimination of the transmitted light and decrease in the maximum brightness.